That sounds like assault with a weapon to me. I don't believe I would beat him down if he did that to one of my kids, pretty sure I would put a bullet in him. One punch can be deadly to an adult, and especially a kid.

I'd suspect that in a fictional comedy setting, the idea of an old(er) geezer that wanders around Wal-Mart looking for children whom can be stealthily whacked in the back of the head would be hilarious.